- Bacteria and food particles stuck in the pet's mouth.
- The pet's diet.
- Eating disorders such as coprophagia (eating feces)
- Kidney disease
- Lung disease
- Mouth tumors
When beginning a regular brushing program use a tooth wipe or finger toothbrush for the first couple of weeks and then begin using a tooth brush once your pet is comfortable with the process.
PetHealth101 is an excellent resource for articles on how to brush your pet's teeth.